We have been studying a model neurosecretory system for the in vitro secretion of norepinephrine in rat heart slices. The slices are incubated in a Na ion-deficient (choline ion) medium and the secretion is Ca ions dependent. Methods and general characteristics of the model have been described in previous reports. It has been shown that neurosecretion has some characteristics of outward transport and some characteristics of synaptic transmission. Moreover, the model is a high capacity system that can secrete 70% of the stored NE within 140 min. In view of the fact that serious questions remain for the establishment of exocytosis as the secretory process in nerve endings, outward transport may be considered a potential mechanism for the physiological secretion of transmitter. In case of malfunction of the storage process, moreover, the norepinephrine in vesicles attached to the synaptic membrane could be transported into the synaptic cleft.